1. Either click 'Edit with Divi' from the Wordpress Pages page, or if you are already on the page and logged into wordpress, click 'Enable the Visual Builder' in the top bar (note: this will only show up if you're logged in to wordpress).
Once on a page, to edit an existing element you can hover over it until you see a grey box. From there, click on the settings/gear icon to edit that specific element. This can be buttons, text, post sliders, etc.
If you wanted to add a new module (like a button underneath for example), you could click the grey plus sign and add another button module to it. You can also easily clone an existing element by clicking the icon with two boxes beside the settings / gear icon.
Divi Builder Help: https://www.elegantthemes.com/documentation/divi/builder/